I was amazed a the size of this mussel shell in a secluded cove and had to get closer to be sure it was exactly what I thought it was. Here it is on my shrub rake, it's 6 inches across. It's probably the Eastern elliptio (Elliptio complanata).
What bothered me though, was that this mussel shell was open and in the sun, with the living tissue parts no longer attached to the shell. While it allowed me to see and photograph its intriguing innards, the fact that it had no future was upsetting.
Since the age of mussels can be determined by counting the rings on their shells, this one looks like it's easily more than twelve years old, and probably closer to twenty.